1. Field of the Invention
The present invention relates to a toner for electrophotography, and more particularly to a toner for forming images for use in image forming apparatuses using electrophotographic methods such as electrostatic copiers and laser beam printers, an to a one-component developer, a two-component developer, an image forming method, an image forming apparatus and a process cartridge using the toner.
2. Discussion of the Background
Since image forming apparatus are now being required to produce more images per unit time, they are demanded to produce images at higher speed. Because of this, they are required to technically have more severe conditions. Particularly for electrophotographic methods being used for on-demand digital printings, wider temperature ranges are needed not to generate offset while producing images having high glossiness.
However, a heat energy per unit time (nip time) providable to a toner for forming images (hereinafter referred to as a “toner”) is less than ever before because of the recent high-speed printing and environment-friendliness. Therefore, a toner is occasionally not fully heated and melted on the surface of a recording medium.
When a toner is not fully melted when fixed, a toner layer on a recording medium is cut into two parts at a point which is not fully melted due to insufficient viscosity. One of the part remains on the recording medium and the other part adheres to a fixing roller. Alternatively, since the toner does not fully adhere to the recording medium, all the toner thereon occasionally adhere to the fixing roller (offset). The toner adhering to the fixing roller is fixed on an undesired place on recoding medium fed next, resulting in ghost images. Namely, when a toner is not fully heated, so-called a cold offset problem occurs.
Even when the cold offset problem does not occur, low-quality images such as images having noticeably deteriorated glossiness are produced.
Therefore, toners including a resin and a release agent having a low softening (melting) point, and a fixation aid are strenuously developed for the purpose of fixing the toner at lower temperature.
For examples, the applicant of the present invention discloses in Japanese published unexamined application No. 2007-72333 specifying a difference between endothermic peaks of a toner before and after heated at 40° C. for 72 hrs, and in Japanese published unexamined application No. 2007-20697 specifying a ratio of an FTIR spectrum of a crystalline polyester resin included in a toner before stored to that thereof after stored 45° C. for 12 hrs to improve low-temperature fixability, heat-resistant storage ability and offset resistance of a toner.
In addition, the applicant discloses in Japanese Patent No. 3478963 specifying dispersion diameters of a colorant and a release agent in a binder resin, respectively and a charge quantity of a toner (charge-up ratio Z (%)=Q20/Q600×100 wherein Q600 is a charge quantity when a toner having a concentration of 5% is mixed with a carrier for 10 min at normal temperature and normal humidity, and Q20 is a charge quantity when the toner is mixed therewith for 20 sec) to improve image density, color reproducibility, offset resistance and charge-up property of the toner.
However, a toner having good low-temperature fixability is typically solidified under an environment of high temperature. Namely, the low-temperature fixability and the heat resistant storage ability of a toner have a trade-off relationship. In other words, toners are required to fix at lower heat energy because image forming apparatuses are required to produce images at higher speed, but toners having good low-temperature fixability do not have sufficient heat resistant storage ability and is difficult to store and transport under an environment of high temperature. Toners having good heat resistant storage ability do not have sufficient low-temperature fixability, resulting in image quality problems such as cold offset.
As disclosed in Japanese Patent No. 3044595, for toners prepared by pulverization methods including a melting process and a kneading process, methods of widening fixable and releasable temperature thereof, in which two or more resins having a different molecular weight or a rheology from each other are heated and kneaded such that a low-molecular-weight resin has toners have low-temperature fixability on base media (recording media) and a polymeric or highly-elastic/highly-viscous resin prevents toners from adhering to fixing rollers and offsetting when fixed at high temperature, are widely used.
However, when two or more resins having noticeably a different molecular weight or a rheology from each other are heated and kneaded to widen fixable temperature of a toner, a shearing strength is not applied to the resins when kneaded due to a difference of the viscosities of the resins, resulting in uneven dispersion thereof. In this case, highly-viscous and low-viscous parts are observed as a sea and island structure, and dispersibilities of a pigment, a release agent or a charge controlling agent noticeably deteriorate, resulting in low fixability of a toner, production of images having uneven image density, foggy images and particularly images having low color saturation.
Particularly, in high-speed printing machines applying less heat energy per unit time, images having low color saturation are noticeably produced, and which is a problem to be immediately solved when using toners prepared by pulverization methods.
Because of these reasons, a need exists for a toner having good storage ability and good fixability, and producing images having good color reproducibility and high image density.